1. Field of the Invention
The invention relates to solid-state color imaging devices, particularly to a solid-state photosensitive device that has a planar array of charge-handling semiconductor photosensors in micro-registration with a multicolor planar array of filter elements. The color imaging devices are particularly useful for solid-state video cameras.
2. Description Relative to the Prior Art
A reliable yet sensitive all-solid-state camera would find abundant utility, including, for example, use in television, card readers, facsimile recorders, picturephones, and character recognition, etc. However, in addition to the problems of drift, misalignment and short tube life, non-solid state color cameras suffer from the complications of having to register separate electron beams and to reduce the effects of electron beam lag. Thus, a relatively simple, efficient solid state color camera which would overcome these problems is still sought.
Color photosensitive devices using solid-state image sensors of various types, for example, charge coupled devices, known as CCDs, and charge coupled imagers known as CCIs, have been proposed for and used in video cameras. To avoid optical complexity and problems with image registration, it is highly desirable that color image sensing occur at a single imaging site, e.g., at a single planar photosensitive array. Difficulty is encountered with such "single-site" color imaging, however, because at least three distinct types of color information must be extracted in order to represent a color image in video signal form.
One known approach to providing a "single-site" color sensing device uses a single, fairly large, image sensor of broad wavelength sensitivity and a cooperating filter disc which passes a series of color filters through the image beam in a repeating sequence. The filter interpositions are synchronized to image scanning, a filter typically being interposed during an entire field scan. Devices operating in this manner are said to produce a "field sequential" color signal. One problem with this approach is that the resulting signal presents the extracted color image information in a time order which is radically different from the time order of, say, the standard NTSC video signal. (The standard NTSC video signal is described in Chapter 16, "Television Transmission", of Transmission Systems For Communications, Revised Third Edition, by Members of the Technical Staff of Bell Telephone Laboratories, Copyright 1964, Bell Telephone Laboratories, Inc.) A further disadvantage is that some of the color image information (e.g., blue image information when a blue filter is interposed) tends to be disproportionately detailed and hence wasteful of sensor capacity in relation to the response characteristics of the human eye.
Certain other proposed approaches to achieving "single-site" color image sensing call for the use of striped color filters superposed on a single, fairly large, image sensor. One such type of image sensor uses filter grids which are angularly superimposed on one another (see U.S. Pat. No. 3,378,633). As a result of image scanning, such image sensors produce a composite signal wherein chrominance information is represented in the form of modulated carrier signals. Such apparatus may be adapted to produce signals in the NTSC format or, if desired, the color image information can be separated by frequency domain techniques. In practice, however, it has proven difficult to produce such sensors economically, particularly where detailed image information is required.
Striped filters which transmit a repeating sequence of three or more spectral bands have also been proposed for use in color imaging. With this arrangement, the filters are typically aligned in one direction and scanning of the image is performed orthogonally to that direction. In effect, elemental sample areas are defined along the filter stripes. With this arrangement, it will be appreciated, sampling for a given color is not uniform for both directions. Additionally, the sampling patterns which result tend to provide a disproportionate quantity of information regarding basic color vectors to which the eye has less resolving power, e.g., "blue" information relative to "green" information.
Another approach to "single-site" color imaging which has been proposed is the "dot" scanning system, as discussed in U.S. Pat. No. 2,683,769 to Banning. That approach generally uses spectrally selective sensor elements which are arranged in triads (red, green and blue elements, respectively). In U.S. Pat. No. 2,755,334, also to Banning, another "dot" scanning system is described having a repeated arrangement of four element groupings (red-, green-, blue- and white-sensitive elements, respectively). Such approaches to color imaging have not been of practical significance, in part because of the difficulty and cost of fabricating the number of individual elements which are required to provide image information having adequate detail.
Many of the problems associated with the prior art "single-site" color imaging processes discussed above are overcome by the approach taken in U.S. Pat. No. 3,971,065, issued July 20, 1976, in the name of B. E. Bayer. In the Bayer approach, color imaging is effected by a single imaging array composed of a large number of relatively small individual luminance and chrominance sensing elements that are distributed according to type (sensitivity) in repeating interlaid patterns, the luminance pattern exhibiting the highest frequency of occurrence--and therefore the highest frequency of image sampling--irrespective of direction across the array.
Preferably, to produce an element array according to the Bayer approach, a solid state sensor array wherein each sensor has a broad wavelength sensitivity is provided with a superposed filter array. Filters which are selectively transparent in the green region of the spectrum are preferably employed in producing luminance-type elements, while filters selectively transparent in the red and blue spectral regions, respectively, are preferably employed in producing chrominance-type elements. (The term "luminance" is herein used in a broad sense to refer to the color vector which is the major contributor of luminance information. The term "chrominance" refers to those color vectors other than the luminance color vectors which provide a basis for defining an image.)
Methods for providing a multicolor filter array are known in the art. For example, U.S. Pat. No. 3,839,039, issued Oct. 1, 1974 to Suzuku et al, shows a multicolor filter which consists of a plurality of monocolor stripe filters laminated together. Each monocolor stripe filter is made by a process comprising exposing a substrate having a photosensitive surface to light through a striped mask, converting the light image into a metallic image, forming a dichroic layer uniformly on top of the metallic image and removing the portion of the dichroic layer associated with the metallic image. U.S. Pat. No. 3,771,857, issued Nov. 13, 1973 to Thomasson et al, shows another multicolor striped filter which consists of a plurality of layers of striped monocolor filters formed successively on top of each other. U.S. Pat. No. 3,623,794, issued Nov. 30, 1971 to S. L. Brown and U.S. Pat. No. 3,619,041, issued Nov. 9, 1971 to D. W. Willoughby show multicolor filters consisting of a lamination of monocolor grating filters comprising photoresist grating patterns filled with dye-vehicle filter materials having preferential absorption in different regions of the visible spectrum.
For one reason or another however, these known methods provide color filter arrays which pose many problems when used with an array of charge-handling semiconductive photosensors. For example, many prior art multicolor filters comprise multiple layers of monocolor filter patterns stacked sequentially on top of each other in order to obtain multicolor filter arrays. However, in order to achieve micro-registration between the filter array and the sensor array, it is desirable for each element in the filter array to be as close as possible to the surface of the underlying photosensor element or elements in the array. This result is most desirably accomplished by producing a relatively thin, single layer multicolor filter array superimposed on the surface of the image sensor. A single layer multicolor filter array substantially reduces the possibility that light rays which pass through a filter element at an angle to the optical axis will strike a photosensor element beneath an adjacent filter element. Further, higher resolution can be obtained by reducing the depth of focus requirements for the optics. These results are not achievable with prior art multiple layers.